Nitrogen cycling at treeline VII. Laboratory Decomposition

To assess spatial and seasonal patterns of pools and fluxes of dissolved inorganic- N (DIN), amino acid- N (AAN) and microbial biomass N (MBN), we conducted in situ soil incubations at four time periods during May 2001 - May 2002: spring thaw, peak growing season, fall senescence and over-winter. The goal was to be consistent in sampling each of these time periods within each mountain range, which was possible due to the 3-4 week lag in phenology (e.g. budbreak or initiation of senescence) between the southernmost and northernmost sites. Sites were sampled in order from south to north. During the spring 2002 sampling period, soils in the Brooks Range thawed prior to those in the White Mts. and the sampling sequence was adjusted to accommodate this. Within each treeline or forested sub-site, a 50 m transect was established parallel with the slope contour of the mountain. Six points were randomly selected along each transect, and soils were sampled near these points for the entire year. Rates of net DIN mineralization and net AAN production were measured using an in situ buried bag technique (Robertson et al., 1999). We used a 6.7 cm diameter steel corer fitted with a perforated, plastic sleeve to collect paired adjacent soil cores, and sampled below the live moss and detritus layers to a depth of 20 cm. The function of the perforated sleeve was to maintain structural integrity of the soil core during sampling. The perforated sleeve containing the intact core was then placed in a 1 mil breathable polyethylene bag followed by a fine mesh bag, gently returned to the original location, covered with litter and left to incubate. Incubation length was 4 weeks for the spring, growing season, and senescence sampling periods, and from September 2001 to early June 2002 for the over-winter sampling period. The second core in each pair was stored on ice and transported to the laboratory in Fairbanks. Soils were rocky at some sites and sampling to 20 cm was not possible; for these samples, the minimum depth of coring was 10 cm. After harvesting each core, the subsequent pit was backfilled with soil to minimize disturbance to adjacent samples. We examined the decomposability of soil organic matter by measuring mass loss over 1 year at all sites using 15.2 x 1.8 cm birch wood tongue depressors (TDs) as a common litter. TDs were oven dried for 24 hours and weighed before placement in the field. During September 2001, three 20 m transects were randomly established at high, mid and low elevations (spaced 10 m apart) at each forest and treeline sub-site. Five TDs were inserted vertically into the soil profile until flush with the surface of the organic layer at 5 m intervals along each transect. In September 2002, TDs were carefully collected, transported to the laboratory in Fairbanks, rinsed, oven-dried and reweighed. Percent mass loss was calculated. The site at Site Summit on Fort Richardson Military Base was closed during fall 2001 and was not sampled.为评估溶解态无机氮（dissolved inorganic-N, DIN）、氨基酸态氮（amino acid-N, AAN）以及微生物生物量氮（microbial biomass N, MBN）的库与通量的空间及季节分布格局，我们于2001年5月至2002年5月期间开展了4个时段的原位土壤培养实验：春季解冻期、生长旺盛期、秋季衰老期与越冬期。本研究的目标是在每个山脉范围内对上述各时段开展一致性采样，而由于最南端与最北端样点间存在3~4周的物候期（例如芽萌动或衰老启动）滞后，该目标得以实现。采样顺序按从南至北的方向进行。在2002年春季采样期间，布鲁克斯山脉（Brooks Range）的土壤解冻早于怀特山脉（White Mts.），因此我们调整了采样顺序以适配这一情况。在每个林线或森林亚样点中，我们沿山地坡等高线设置了一条50 m的样带。沿每条样带随机选取6个采样点，全年在这些点位附近采集土壤样品。我们采用原位埋袋法（Robertson等，1999）测定了净溶解态无机氮矿化速率与净氨基酸态氮生成速率。我们使用直径6.7 cm的钢质取土钻搭配带孔塑料套管采集成对的相邻土芯，并在活体苔藓与碎屑层下方取样，采样深度达20 cm。带孔套管的作用是在采样过程中维持土芯的结构完整性。将装有完整土芯的带孔套管放入1 mil厚度的透气聚乙烯袋中，再套入细网袋，随后轻柔放回原采样位置，覆盖枯落物后进行原位培养。春季、生长季及衰老期采样的培养时长为4周，越冬期采样的培养时长为2001年9月至2002年6月初。每对土芯中的另一份样品置于冰上保存，随后运往费尔班克斯的实验室。部分样点的土壤含有石块，无法达到20 cm的采样深度；对于这类样品，最小采样深度为10 cm。每份土芯采集完成后，用土壤回填后续形成的土坑，以尽可能降低对相邻样品的干扰。我们以15.2×1.8 cm的桦木压舌板（birch wood tongue depressors, TDs）作为标准枯落物，通过测定其1年内的质量损失率，分析了各站点土壤有机质的可分解性。压舌板先经烘箱干燥24小时并称重，随后投入野外实验。2001年9月，我们在每个森林与林线亚样点的高、中、低海拔（间隔10 m）区域随机设置3条20 m的样带。沿每条样带以5 m的间隔，将5根压舌板垂直插入土壤剖面，直至与有机层表面齐平。2002年9月，我们小心回收压舌板，运往费尔班克斯的实验室后进行清洗、烘箱干燥并重新称重，以此计算质量损失百分比。理查森堡军事基地的萨米特站点（Site Summit）在2001年秋季期间被关闭，未参与采样。